1. Field of the Invention
The present invention relates to an improved two-stage process for the preparation of macrocyclic esters or lactones from dicarboxylic acids or dicarboxylic acid dialkyl esters and diols or from hydroxycarboxylic acids using a novel catalyst. The present invention also relates to the second stage of the process separately.
2. Description of the Background
Macrocyclic esters and lactones play an important role in the fragrance industry because of their known musk or ambergris note and also as fixatives. The 13- to 18-membered ring systems are the most widely used.
It is known that macrocyclic lactones can be prepared by a two-stage reaction in which a linear oligoester or polyester is first prepared, and then cyclized by thermal depolymerization. The depolymerizing cyclization is described, for example, in J. Am. Chem. Soc. 57 (1935), 929-34 and in U.S. Pat. No. 4,175,321.
The processes for the preparation of the oligoesters or polyesters in the first stage, employ dicarboxylic acids and diols as starting materials, and usually proceed at from 120 to 200.degree. C., and in most cases without catalyst. For example, the process of DE 32 25 431 uses only the acidity of the dicarboxylic acid, omitting an acidic catalyst, and accepts with long reaction times are required. The processes of DE-A 28 40 905 and of U.S. Pat. No. 4,393,223 employ an acidic fixed-bed catalyst. Prior to the depolymerization stage, it is, however, necessary to add a further catalyst because the effectiveness of the acidic fixed-bed catalyst in the subsequent depolymerization stage is limited.
The cyclizing depolymerization of the second stage is described, for example, in J. Am. Chem. Soc. 57 (1935), 929-34 and in U.S. Pat. No. 4,175,321, and is carried out in the presence of a catalyst and generally at temperatures of from 200 to 300.degree. C. The macrocycles which form have a relatively high vapor pressure under these conditions, whereby it is possible to remove them from the reaction mixture or the reactor by distillation. A large number of catalysts for this stage are known, for example Lewis acids, alkali metal alkoxides and alkaline earth metal alkoxides, oxides, hydroxides and salts of, for example, halohydric acids, nitric acid, carbonic acid, boric acid and of carboxylic acids. Some of the parent acids of the salts are also suitable as a catalyst. Suitable cationic constituents of the salts are alkali metals, alkaline earth metals and heavy metals, such as iron, copper, lead, zinc, tin, nickel, titanium and zirconium.
In processes in which the starting materials are dicarboxylic acid dialkyl esters and diols, and which thus prepare the oligoesters or polyesters by transesterification, catalysts which have proven successful are alkyl stannates, titanates and zirconates (see DE 32 254 31, DE 28 409 05). They are also suitable as catalysts for the depolymerizing cyclization. These compounds do, however, have a relatively high vapor pressure and can only be separated off from the macrocyclic esters or lactones with difficulty. Since they are also transesterification catalysts, their presence even in concentrations of a few ppm during distillation or storage leads to linear oligomers or polymers.
Thus, a need exists for a process for producing macrocyclic esters or lactones, which overcomes the above disadvantages.